Used Posi Differential Rebuild - Junkyard Builder

Over the years, we thought we knew a few tricks about how to set up a typical Eaton 12-bolt, clutch-style limited slip, better known as a Posi. But we’ve learned a slick trick from Tom at Tom’s Differentials that makes that clutch Posi not only work much better but also last longer. It turns out this differential rebuild procedure can also be applied to other new or used clutch-style limited slips, such as the Ford 8.8 and the Ford 9-inch Traction-Lok. While this story is focused on the GM 12-bolt, the differential rebuild procedure is the same for the 8.2 and 8.5 10-bolts, although some part numbers will change. A typical, new Eaton clutch-style limited slip relies on a thin side-gear shim and a set of coil springs to preload the clutches, which produces an adequate amount of torque to drive both rear wheels. This works for a while until the clutches and gears wear. This added clearance reduces the torque capacity and usually results in tire spin. Tom’s Differentials prefers to custom-build these units, beginning with seating the new gears and clutches by rotating the Posi 50 turns in each direction, then adding a thicker shim package. This requires repeating the process of shimming and spinning until eventually the entire differential is set to the proper clutch preload. This blueprinting process also eliminates the need for preload springs, which, according to Tom’s, reduces the load on the clutches and increases their durability.

This differential rebuild procedure establishes a preload you can feel as you turn the case on the shaft. Tom says you’ll know when you have it right. We took a few new parts and our slipping, used Posi over to Moore Automotive, where Tim Moore went through the differential rebuild procedure. If you like moving pictures, you can watch a how-to video that Tom’s has produced (search “Tom’s Differential Posi Rebuild” on YouTube). It’s important to mention that this is a procedure Tom developed. He could have kept it to himself but chose to share it with other car crafters. Armed with this knowledge, you can now do the labor yourself and save a little money you can then use to buy more parts from Tom. It’s a concept that deserves support.

Parts List

Description

PN

Source

Price

30-spline side gears

C12-1017

Tom’s Differentials

$185.00

33-spline side gears

C12-1017-33

Tom’s Differentials

220.00

Hard cross-shaft

TCS-C12

Tom’s Differentials

30.00

Thrust washer

C12-TW

Tom’s Differentials

4.75 (ea.)

Tom’s shim kit

C12-PSS

Tom’s Differentials

20.00

18-disc plate kit

C12-PP 18S

Tom’s Differentials

110.50

Bare 30-spline case

Call

Tom’s Differentials

315.00

Tuned Eaton 30-spline

Call

Tom’s Differentials

885.00

Stock Eaton 30-spline

Call

Tom’s Differentials

Call

Select-fit C-clip

Call

Tom’s Differentials

6.00 (ea.)

Lucas 85W-140 lube

N/A

Tom’s Differentials

7.00 / qt.

GM Posi lube

G-PA

Tom’s Differentials

11.35

The Other Guys

This technique will also work for other Eaton clutch-type limited slips, as used in the GM 8.2 and 8.5 10-bolts as well as the Ford 8.8 Eaton unit. In fact, the Eaton 8.8 uses the same clutches as the GM 12-bolt, and Tom’s offers hardened 8620 steel cross-shafts for these units. The Ford 9-inch Traction-Lok is also rebuildable with new clutches, gears, hardened cross-shafts, and a similar shim pack from Tom’s, so you have plenty of opportunity to upgrade a used limited slip. End